PLANT NUTRITION 265 



:i r;i|)i(l ;i\;iil;il)ilit\ of nitiogcn for plant growth. Excessive (juantitics 

 ol nitrates arc nndcsirablc Irom the standpoint of permanent soil 

 ieitility because thc) lend to inhibit the processes of nitrification and 

 nitrogen fixation. Ihe residual effect of sodiimi, potassium, and cal- 

 cium nitrates is to decrease soil acidity, whereas ammonium nitrate 

 increases soil acidity. 



Ammonium compounds. Of all the nitrogenous fertilizer materials, 

 ammonium sulfate is the most widely produced and used. Much 

 of it is prepared by the neutralization of dilute sulfuric acid with am- 

 monia, a by-product of destructive distillation of coal. In the gypsimi 

 process, another common method for the preparation of ammonium 

 sulfate, ammonia and carbon dioxide are passed into a suspension of 

 calciimi sulfate, forming ammonium sulfate and calcium carbonate. 



2NH3 + CaS04 + CO2 -f H2O -> (NH4)2S04 + CaCO, 



Ammonium sulfate tends to increase soil acidity, and soils fertilized 

 with this source of nitrogen must be limed more frequently than those 

 treated with sodium nitrate. Aqueous solutions of ammonium sulfate 

 are acid, since the compound is a salt of the weak base NH4OH and 

 the strong acid sulfuric. In addition soil acidity increases further as 

 a result of nitrification. The reactions can be shown as follows: 



NH4+ -> H+ + NH, -> H+ + NOr 



A j)()pular method of explaining thc acidifying effect ol amuionium 

 sulfate is to state that nitric and sulfuric acids are formed. Ammonium 

 sulfate is a good source of nitrogen for most crops but not so satisfac- 

 tory as nitrates for acid-sensitive plants such as beets, barley, and 

 wheat. Other ammonium compounds sometimes used as nitrogenous 

 fertilizers include ammoniimi chloride, ammonium nitrate, and several 

 kinds of ammonium phosphates. Basic compounds such as lime or 

 basic slag should not be mixed with ammonium salts, since a reaction 

 takes place which liberates nitrogen as ammonia. 



(NH4)2S04 + CaO -^ CaS04 + 2NH3 + H2O 



Anhydrous ammonia and various solutions with a high ammonium 

 content are used as sources of nitrogen by the manufacturers of mixed 

 fertilizers. These sources of ammonia have been derived either from 

 the air by direct synthesis or from the by-products of other reactions. 

 They represent the cheapest available source of nitrogen. Anhydrous 

 ammonia or ammonia solutions are utilized by mixing them with 

 superphosphate. A series of reactions takes place which results in the 

 formation of monoammonium phosphate, ammonium sulfate, and 



